In a screw fluid machine for compressing and expanding a fluid by male and female screw rotors meshed with each other, for example in a screw compressor for compressing the fluid by the screw rotors, pressure of the compressed fluid generates a thrust load of pressing the screw rotors toward the suction side in the axial direction. A conventional screw compressor has a problem that in a case where discharge pressure is increased, the life of a thrust bearing for receiving this thrust load is shortened.
Even when a plurality of thrust bearings is provided for a rotor shaft, the load is actually concentrated onto one of the thrust bearings. Thus, the life of the thrust bearing cannot be extended.
Patent Documents 1 to 3 describe the invention in which, in order to reduce loading of a thrust bearing of a screw compressor, a piston fitted to a cylinder is provided at an end of a rotor shaft on the suction side and the rotor shaft is pressed in the direction opposite to a thrust load by pressure of a fluid introduced into the cylinder.
However, the piston of each of the screw compressors is rotated together with the rotor shaft inside the cylinder. Thus, there is a problem that friction between the piston and the cylinder or viscous resistance of the fluid in the cylinder increases a rotational load of the rotor shaft so as to lower energy efficiency. There is also a possibility of causing a fault that a seal between the piston and the cylinder is worn away, so that the pressure fluid in the cylinder is leaked out.    [Patent Document 1] Japanese Patent Laid-Open No. 2002-317782    [Patent Document 2] Japanese Patent Laid-Open No. 2004-339994    [Patent Document 3] Japanese Patent No. 3766725